A known hydraulic brake apparatus for a vehicle including an auxiliary hydraulic pressure source and a pressure control valve, in addition to a master cylinder, is disclosed in JP1997-315288A (which is referred to as reference 1 hereinbelow). The hydraulic brake apparatus according to reference 1 is structured so that hydraulic pressure generated at a pressure chamber of a master cylinder is supplied to wheel cylinders of a first brake circuit and that hydraulic pressure of an auxiliary hydraulic pressure source is controlled by a pressure control valve in response to an output pressure of the master cylinder and then, the controlled hydraulic pressure is supplied to wheel cylinders of a second brake circuit.
The hydraulic brake apparatus according to reference 1 includes the master cylinder piston having first and second pistons. The master cylinder pistons pressurize the hydraulic pressure at the pressure chamber. The first piston is connected to a brake operation member and the second piston is fitted at an outer circumference of the first piston so as to be slidable in an axial direction thereof. In a case where the hydraulic pressure is normally supplied from the pressure control valve to the master cylinder, the second piston receives the output hydraulic pressure generated by the pressure control valve at a back portion (a portion corresponding to the brake operation member) and generates thrusting force which functions as driving assist force of the mater cylinder pistons. The output pressure of the pressure control valve is supplied to the wheel cylinders via electromagnetic valves controlling the hydraulic pressure.
According to the hydraulic brake apparatus in reference 1, the output hydraulic pressure of the master cylinder is determined depending on an operating stroke of the brake operation member, and not on an operating speed of the brake operation member. However, the pressure control valve is configured so that a control piston which receives the output hydraulic pressure of the master cylinder moves a spool to a position in response to the output pressure and the pressure control valve controls and outputs the hydraulic pressure supplied from the auxiliary hydraulic pressure source by the movement of the spool. Thus, because an influence of an orifice effect occurs at an inside of the electromagnetic valves or the like for controlling hydraulic pressure, a response to the operation of the brake operation member is delayed. As a result, when the brake operation member is operated fast, pressurization of the wheel cylinder pressure is delayed in the circuit at which the pressure control valve is provided and the operating stroke of the brake operation member relative to the wheel cylinder pressure becomes longer than when the brake operation member is operated slower.
In other words, a hydraulic pressure balance between the first brake circuit applying the output hydraulic pressure of the master cylinder and the second brake circuit applying the output hydraulic pressure of the pressure control valve changes depending on the operating speed of the brake operation member (brake pedal depressing speed). For example, the brake operation feeling and brake effectiveness vary depending on the way a driver depresses the brake pedal. Accordingly, the driver feels that the brake is not responding to his/her operation at an initial operation, or that the brake is responding to his/her operation with delay. Further, the brake effectiveness is deteriorated by the delay of pressurization of the wheel cylinders of the second circuit.
In order to improve the brake operation feeling and the brake effectiveness, the output pressure of the pressure control valve may be increased relative to the output pressure of the master cylinder, however, a thickness of a cylinder body may be increased for pressure resistance, which results in increasing a size of the cylinder body.
A need thus exists for a hydraulic brake apparatus which is not susceptible to the drawback mentioned above.